The prior art describes fibers coated with metal and composites produced therefrom. For example, in commonly assigned, U.S. application of Morin, Ser. No. 650,583, filed Sept. 12, 1984, and now U.S. Pat. No. 4,661,403, graphite and other semimetallic fibers coated with thin, uniform and firmly adherent electrically conductive layers of metals, including nickel and copper are described. In Morin, Ser. No. 507,603, filed June 24, 1986, now U.S. Pat. No. 4,680,093, their use in metal composites is described. In Iwaskow and Crum, Ser. No. 358,549, filed Mar. 16, 1982, now U.S. Pat. No. 4,752,415, their use in polymeric composites is described. In Morin, Ser. No. 630,709, filed July 13, 1984, now allowed, their use in radar reflecting chaff is described. In Luxon, Ser. No. 869,518, filed June 2, 1986, their use in elongated injection molding granules is described. However, such coatings, while uniform and firmly adherent and electrically conductive, are relatively thin being of the order of 0.25 to 1.0 microns, more normally 0.5 microns in thickness. As such, they do not render themselves useful in certain applications, particularly as starting materials for direct consolidation into composites. It would be desirable to produce such fibers being uniformly and somewhat more relatively thick.
Other prior art, however, which describes thicker coatings, do not have good uniformity of coating thickness either around the fiber circumference, from fiber to fiber throughout the tow, or along the length of the tow. Some prior art materials have heavily coated fibers on the outside of the tow while inner fibers remain uncoated. Other prior art materials contain fibers which are plated together in groups leaving a deficiency of copper in the inside of each agglomerate.
Generally, such features are disadvantageous because they give uneven or irregular properties to any material made from them. In particular, if copper coated fibers are formed into copper matrix composites by consolidating under heat and pressure, a lack of uniformity of the coating thickness will result in an irregular distribution of the base fiber throughout the resulting copper matrix. The presence of uncoated fibers and agglomerates of fibers leads to voids in the matrix in such a composite. Such an uneven distribution of fibers and voids cause poor mechanical properties as well as uneven thermal or electrical conductivity. Agglomerations of fibers can also cause the tow of fibers to be difficult to spread and can thereby make difficult the production of thin composites. In addition, the presence of agglomerations can make the tow of fibers less flexible and so make production of filament wound or woven materials difficult.
Among the prior art proposals to produce copper coated fibers, in particular copper coated carbon fibers, electrolytic plating, electroless plating, a combination of electroless followed by electrolytic plating, cementation, and ion plating have all been tried.
As to electrolytic deposition, B. W. Howlett et al. (Proceedings of the International Conference organized by The Plastics Institute, Unwin Bros., Ltd., Surrey, February 1971, pp. 99-106) states that "a very satisfactory method has been developed for the continuous electro-deposition . . . of copper . . . onto tows containing 1000 fibers". The method is only described as electrolytic deposition via an "experimentally formulated cyanate solution". However, their photographs of the fibers show very uneven thicknesses of coating from fiber to fiber, as well as around the circumference of the fibers. Generally, the literature teaches that direct electrolytic deposition of copper gives rough, uneven coatings. For example, M. Sakai et al., Japan Institute of Metals Journal, 43 181-189, 1979 found "extremely severe hills and valleys in the surface of the electrocrystallized copper" when a copper sulfate (200 g/l), sulfuric acid (60 g/l) plating solution was used. An accompanying photograph shows that the coating thickness varies from fiber to fiber as well. Japanese Patent Disclosure No. 1985-SHOWA 60-17,095 found that in carbon fiber tows plated from a copper sulfate plating bath, that "knitting is difficult because of poor slidability due to the roughness of the copper-plating layer". The patent reports an improvement in slidability when a commercially prepared brightner, UBAC-1, and chloride ions were added to the plating solution. However, no documentation of the coating structure is provided, and when repeated herein, unsuitable coatings were obtained as shown in the comparative examples to follow.
As to electroless deposition, attempts to coat carbon fibers with copper using an electroless copper plating process is reported by several workers. Some reports are vague and insufficiently detailed, for example U.S. Pat. No. 3,550,247 states that "a large bundle of fibers may be coated with metal substantially uniformly", but does not give a specific procedure for plating copper. A report by N. C. W. Judd (Composites, Dec. 1970, p.345) used a proprietary electroless solution to deposit copper onto carbon fibers, but poor coating was obtained. It was stated that "coverage of the fibers at the center of the tow was not achieved." Other reports have used electroless plating to coat carbon fibers with copper. V. N. Sakovich et al. (Fiz. Khim. Obrab. Mater. 1975(2) 112-115 (1975)) precipitated copper by treating the fiber surfaces with boiling water, stannous chloride solution, and palladous chloride solution. Then "a number of electrolytes in which formaldehyde solution was used as a reducing agent were tested". They report that "a bright coating, uniform in thickness on all fibers of the tow, was obtained from the following electrolyte: A:170 g/l Seignette salt (potassium sodium tartrate), 50 g/l sodium hydroxide, 30 g/l sodium carbonate; B: 40 percent formaldehyde solution; A:B=5:1 (ph 12.3, temperature 20C)". It was reported that "the average precipitation rate of the coating was 0.03 micron/minute for a charge density of the electrolyte of 0.6 dm.sup.2 /l. When repeated herein, the coating produced was poor as will be shown. German (Federal Republic of Germany) Pat. No. 2,658,234 describes a process "for coating individual fibers of a bundle, in which the fiber bundle is exposed to a reaction solution from which, by a currentless chemical means, a substance is deposited onto the fiber." In order to assure individualization of the fibers of the tow, "a portion of the fiber bundle is loosely suspended, and . . . the reaction solution is fed along the fibers of this portion". It is stated that "Because of the spreading of the fibers, the reaction solution can reach the surfaces of all the fibers. The fibers can thus be uniformly coated over both their length and their cross section". In one sample embodiment, it is reported that "it is possible . . . to uniformly deposit a 1 micron copper layer on the individual fibers of the bundle". However, when repeated herein, unsuitable coatings were produced as will be shown in the comparative examples.
As to a cementation process, B. C. Pai et al. (Journal of Materials Science, Letters, 15 1860-1863 (1980)) reported on the use of a cementation process to deposit copper and other metals on carbon fibers. It was disclosed that whereas nickel and cobalt coat the fibers uniformly, "copper coating takes place by bridging . . . isolated precipitates". FIGS. 3 through 7 in this reference show irregular thickness copper coatings
As to ion plating or vacuum deposition, the prior art discloses the use of ion plating or vacuum deposition to form "an assembly . . . of a plurality of carbon fibers each coated with a matrix metal layer, the fibers having bonded points at the metal layers". This process does not form individualized fibers. Japanese patent disclosure No. 1982-SHOWA 57-57,851 discloses use of ion plating to coat fabrics of carbon fibers. However, the goal of this process is not to form individualized fibers as the present invention requires and forms.
As to electroless followed by electrolytic deposition, A. M. Kuz'min et al. (USSR Pat. No. 489,585, and Fiz. Khim. Obrab. Mater., 1975(5) 101-106) reported carbon fiber tows coated with copper prepared by depositing a layer of copper using an electroless process and subsequently depositing further copper electrolytically. It was stated that the process gives a material in which "all elementary fibers are covered with a uniform layer of metal in the process of chemical precipitation . . . the filling with metal during the electrolytic build-up of the layer also proceeds uniformly". When repeated herein, however, uniform layers were not obtained. U.S. Pat. No. 3,495,940 discloses a process "comprising the steps of (1) forming thin films of sensitizing and activating metals on the surface of an organic fiber, (2) electrolessly plating, a thin layer of copper on the activated surface, (3) electrodepositing additional copper on top of said copper layer". The process was used to form copper coatings comprising 10 to 50% by weight of said organic fiber. This is a coating thickness of only 0.048 to 0.24 microns for an organic fiber having a density of 1.25 g/cm.sup.3, too thin to provide the advantages of the relatively thick copper coatings required herein. The patentee, moreover, used this fiber to electrically heat the organic fiber to the point that the fiber carbonizes and graphitizes to a carbon fiber. Since such processes reduce the volume of the basic fiber, the carbon fiber is not attached to the metal coating. No discussion was included concerning the way in which the fiber tow was handled. Japanese Patent Publication No. 1984-59 SHOWA-53640 discloses "compound plating, which is a combination of electroless plating and electroplating". In this method, the carbon fibers are held under tension on a frame and then coated. The object of the process was to produce a "prepreg" of copper and carbon fibers and not to produce individualized coated fibers. Composites produced by hot pressing the prepregs were unsuitable because they had non-uniform distributions of fibers as evidenced by photographs of cross sections presented therein. Kuz'min, cited above, used a process of air oxidation or nitric acid etching, followed by electroless plating, and then electrolytic plating. While the bath components are given, no specific bath formulation is given. In addition, no fiber handling techniques are noted. EPO No. 0-156-432 discloses the coating of single optical fibers with a combination of electroless followed by electrolytic plating. "In a favorable embodiment . . . the fiber . . . is electroless plated continuously in a first step and is electroplated continuously with a metal layer in a second step . . . . the fiber is passed successively through a number of deposition baths alternated by rinsing baths . . . . In a particularly advantageous embodiment . . . a layer of metal is provided continuously by electrodeposition on the electrically conductive layer in at least two successive steps, in which between the successive steps the current density is increased and the metal-coated fiber is electrically contacted". No plating of copper is actually exemplified.
Also disclosed in the prior art, are composites that have a copper matrix reinforced with fibers, these having been formed by a number of techniques. For example, tows of reinforcing fibers can be impregnated with a slurry of copper powder and then consolidated under heat and pressure. Sometimes this process may involve sandwiching the fibers between copper foils. Such methods, however, have several disadvantages. Since copper does not spontaneously wet some fibers, especially carbon fibers, additives, for example, titanium, must be provided in the copper to permit wetting. However, such additives reduce the thermal and electrical conductivity of pure copper. Such composites also have unevenly distributed reinforcing fibers.
To avoid having to wet the carbon fiber with liquid copper it is advantageous to coat the fibers with copper individually and consolidate the coated fibers under heat and pressure. This general procedure has been used by a number of workers. However, since the ability to coat all the fibers of a tow uniformly has been limited, it has not been possible until the present invention to create uniform distribution of reinforcing fibers in a copper matrix composite. In addtion, it is not uncommon for hot pressing to be done at very high temperatures (around 900.degree. C.) which can cause dewetting of a copper coating from a carbon fiber and lead to defects at the copper/carbon interface. The present invention avoids this drawback, too.